Method of preparing a lithographic printing plate



Unite States Patent 7 METHOD OF PREPARING A; LITHO GRAPHIC PRINTING PLATE Application October-25, 1955, Serial'No. 542,782

Claims. (Cl. 101-1492) N 0'. Drawing.

' This invention relates to lithography-and particularly -toa methodfor improving :the-ink receptivity of lithographic printing plates.

Lithographic printingplates have a hydrophilic surface which repels lithographic printing ink. -ln-use, an image is' formed'on the surface of-the plate which accepts ink and which printsthe-imagein a lithographic printing press. Lithographicrpaper;printing plates are usually coated with a mixture of a hardenedhydrophilic organic colloid and an inert pigment. Suitable-printing plates haveapaper support which has beencoated with a mix- :ture of well hardened c-aseinand a pigment such as' clay, barium-sulfate, etc. If'the casein coated paper has a layer containing gelatin and pigment, it'may be treated -to=render the surface less absorptive.

'It is known that the ink'receptive image'can'be formed photographically on a separate sheet and transferred to the lithographic-printing plate. A'processof'this-type is described in Clarkand Cowden- U. S. application Serial No. 267,482, filed January 21, 1952, now U. -S.'Patent No.'-2;763,553. These processeshave defects, however, mainly because -the-transferred image is not sufliciently durable in the printing operationand is not sufliciently 'inlc receptive.

It is therefore an object of the present invention to provide a method, for providing a lithographic printing plate having good ink receptivity and durability for printing. Other objects will appear from the following description of our invention.

These objects are accomplished by transferring an unhardened gelatin image to a lithographic printing plate which will repel greasy printing inks when wet with Water, Wetting the surface of the printing plate and the transferred gelatin image with a solution of tannic acid and thiourea after which the printing plate is wet with water and run in a lithographic printing press in the customary way.

A suitable paper printing plate has a melamine-formaldehyde sized paper stock of medium weight coated with a mixture of gelatin and baryta containing a gelatin hardening agent such as formaldehyde or aluminum chloride. A suitable plate may also be formed of -a sized paper stock of medium weight rendered water repellant with a casein coating having about 0.15 to .6 g. of casein per square foot of surface. The casein coating may contain an inert pigment such as clay or baryta and may be treated, if desired, with 'a cuprammonium solution or a salt solution such as zinc acetate before, after or during application to the casein-pigment coat of a thin coating of a hydrophilic colloid such as sodium algin-ate, as described in U. S. Patent 2,534,650.

The photographic gelatin image which is to be transferred to the printing plate may be formed in various ways, but preferably is made and transferred as described in Yutzy and Yackel U. S. Patents 2,596,756 and 2,716,059. As described in these patents, a substantially unhardened gel'atino-silver halide, which contains a tanice ning developing agent or both a tanning and a non-tanning developing agent is exposed to a line or halftone image and developed to form a tanned or hardened gelatin image. The tanned gelatin image may also be formed by-use of an external developing agent not incorporated in the emulsion layer. After a tanned or hardened gelatin image is formed in this way the remaining gelatin which is. unhardenedis transferred to an absorbent support, e. g. paper, cloth or wood, or in the present case, to the hydrophilic surfacev of. a lithographic printing plate which .will repelgre'asy, printing inks when wet with water.

The gelatin image. transferred in this way is soft and hydrophilic and must be rendered ink receptive as well -as sufiiciently vhard and, tough to withstand abrasionand printing onthe lithographic press. -We have found that this maybe accomplished by treating the plate after the gelatinimage has been transferred to it, with a solution containing tannic acid and thiourea and other agents if desired. In addition to tannic acid and thiourea, the solution used to treat the plate may contain ascorbic acid, aluminum chloride sodium bisulfate, potassium. bichromate, hydrochloric acid or chromic chloride.

The following examples illustrate our invention.

Example 1 An image of a typewritten document was formed in a light-sensitive emulsion in the manner described in Yutzy and Yackel U. SfPatent 2,716,059 and the unha-rdened gelatin of the sensitive material was transferred to a lithographic printing plate instead of to a sheet of absorbent paper. A printing plate of the type described above was used, in this case .a plate sold under the trade name Remington Rand 'Plastiplate. The transferred gelatin image was .swabbed' for 20 seconds with a solution of tannic acid and thiourea in water containing 10% tannic acid and 5% thiourea. The plate was heated for 20 .secends with .lightfrom two. Photoflood lamps about one foot away and was then moistened. with water or conventional lithographic etching solution, placed on an offset press and run under standard printing conditions. A run of, 500 copies was :obtained.

7 Example 2 A typewritten document was exposed to the sensitive material described in Example 1 and the transfer made to a lithographic printing plate such as described above, in this case a plate sold under the trade name Polychrome V Kote Plate. The transferred image was swabbed with a solution in water of 4% tannic acid, 4% sodium bisulfate and 4% thiourea. The treated plate was then placed on an offset press and to copies were made in the usual way.

The use of thiourea in our formulas is particularly important since ink receptivity of the plate on the press is dependent on this material. The usual gelatin tanning agents are ineffective in the presence of thiourea but tannic acid is effective to harden the gelatin when thiourea is present.

One disadvantage of transferring the gelatin image from the matrix as described in Examples 1 and 2, to the lithographic plate is that certain plates cause the matrix to frill and transfer background gelatin, thus preventing satisfactory offset press results. This appears to be caused by the acidity of the surface of the offset plate which frequently has a surface pH of 4 to 5 whereas the matrix pH is about 9 to 11 at the time of transfer. However, matrix frill may be caused by high pigment-tobinder ratios in the surface coating of the master or inclusions of excessive amounts of gelatin reactors such as formaldehyde.

We have found that this undesirable frill may be Example 3 A lithographic printing plate such as described above, for example, a plate known as Quality Direct Image Plate sold by Polychrome Corporation of Yonkers, New York, or 7900 Series Plastiplate sold by Remington Rand Division of Sperry-Rand Corporation was hand rubbed with a 2% borax solution and dried. A transfer of a typewritten image was then made to the plate as described in Example 1 and the plate was swabbed with the solution of tannic acid, sodium bisulfate and thiourea described in Example 2. The plate was then run on' an offset press and at least 100 copies were obtained.

It will be understood that the examples and modifications described herein are illustrative only and that our invention is to be taken as limited only by the scope of the appended claims.

What we claim is:

l. The method of preparing a lithographic printing plate having good lithographic ink receptivity and durability, which comprises forming by exposure and development in a 'gelatino-silver halide emulsion layer of unhardened gelatin, a hardened gelatin image, transferring the unhardened gelatin portions of said emulsion layer to a lithographic printing plate which will repel greasy printing inks when wet with water, wetting the surface of the printing plate and the transferred gelatin image with a solution containing approximately 4% to tannic acid and approximately 4% to 5% thiourea, and

ability, which comprises forming by exposure and development in a gelatino-silver halide emulsion layer-of unhardened gelatin, a hardened gelatin image, transferring the unhardened gelatin portions of said emulsion layer to a lithographic printing plate which will repel greasy printing inks when wet with water, wetting the surface of the printing plate and the transferred gelatin image with a solution containing approximately 4% to 10% tannic acid, approximately 4% to 5% thiourea and approximately 4% sodium bisulfate, and then wetting the surface of the printing plate with water and placing it on an offset printing press.

3. The method of preparing a lithographic printing plate having good lithographic ink receptivity and durability, which comprises forming by exposure and development in a gelatino-silver halide emulsion layer of unhardened gelatin, a hardened gelatin image, transferring the unhardened gelatin portions of said emulsion layer to a lithographic printing plate which will repel greasy printing inks when wet with water, wetting the surface of the printing plate and the transferred gelatin image Withfan aqueous solution containing 4% tannic acid, 4%,thiourea and 4% sodium bisulfate, and then wetting the surface of the printing plate with water and placing it on anroifset printing press. I

'4. The method of preparing a lithographic printing plate having good lithographic ink receptivity and durability, which comprises forming by exposure and development in a gelatino-silver halide emulsion layer of unhardened gelatin, a hardened gelatin image, washing the acidic surface of a lithographic printing plate which will repel greasy printing inks when wet with ater, with an alkaline solution to neutralize its acidity, transferring the unhardened gelatin portions of said emulsion layer to said printing plate, wetting the surface of the printing plate and me transferred gelatin image with an aqueous solution containing 4% tannic acid, 4% thiou'rea and 4% sodium bisulfate, and then wetting the surface of the printing plate with water and placing it on an offset printing press.

5. An aqueous solution for treating a lithographic printing plate having a gelatin image thereon, comprising approximately 4% to 10% tannic acid, approximately 4% to 5% thiourea and approximately 4% sodium bisulfate.

No references cited. 

1. THE METHOD OF PREPARING A LITHOGRAPIC PRINTING PLATE HAVING GOOD LITHOGRAPHIC INK RECEPTIVITY AND DURABILITY, WHICH COMPRISES FORMING BY EXPOSURE AND DEVELOPMENT IN A GELATINO-SILVER HALIDE EMULSION LAYER OF UNHARDENED GELATIN, A HARDENED GETATIN IMAGE, TRANSFERRING THE UNHARDENED GELATIN PORTIONS OF SAID EMULSION LAYER TO A LITHOGRAPHIC PRINTING PLATE WHICH WILL REPEL GREASY PRINTING INKS WHEN WET WITH WATER, WETTING THE SURFACE OF THE PRINTING PLATE AND THE TRANSFERRED GELATIN IMAGE WITH A SOLUTION CONTAINING APPROXIMATELY 4% TO 10% TANNIC ACID AND APPROXIMATELY 4% TO 5% THIOUREA, AND THEN WETTING THE SURFACE OF THE PRINTING PLATE WITH WATER AND PLACING IT ON AN OFFSET PRINTING PRESS. 